1. Field of the Invention
The present invention relates to a thermosetting resin composition, a prepreg using this composition as a matrix resin, a cured resin product and a fiber-reinforced plastics material having excellent characteristics such as high toughness, high elongation, high modulus, low internal stress characteristics and furthermore, high heat resistance and low water absorption.
2. Description of the Prior Art
Thermosetting resins have been widely used in a variety of industrial fields such as molding, lamination and adhesion by utilizing their excellent mechanical characteristics and chemical resistance. Especially for fiber reinforced composite materials where a reinforcing fiber and a matrix resin are essential constitutional elements, a variety of thermosetting resins, especially epoxy resins are widely used. However, there exists a defect that cured thermosetting resins are generally brittle and accordingly problems arise from the resulting poor impact resistance and low elongation at break of the cured products. Above all, when these resins are used as structural materials in, for example, aircraft and automobiles, poor impact resistance becomes a great problem.
On the other hand, in the fields of semiconductors such as condensers, diodes, transistors and integrated circuits such as IC, LSI, especially as sealing materials protecting mechanically and electrically semiconductors from outer environments, epoxy resins are mainly used at the present time. However, in these applications, there also exists a problem that internal stress is generated due to a difference in linear expansion coefficient between epoxy resins and silicon chips or lead frames or due to strain on curing. This internal stress causes defects and cracks in protective films for semiconductor elements and furthermore, cracks in semiconductor elements themselves. Moreover, this internal stress causes strains in lead frames, which tends to cause poor insulation between frames.
A variety of trials shown below have been carried out to improve these defects, especially the brittleness of thermoset resins.
(1) Toughness of the cured product can be improved by incorporating in an epoxy or phenol resin composition a rubbery polymer having terminal functional groups (e.g., carboxyl group-terminated butadieneacrylonitrile rubbers). However, in this trial, there existed a defect that elastic modulus (especially, elastic modulus at higher temperature) decreased remarkably.
(2) A highly heat-resistant thermoplastic resin was incorporated in an epoxy resin composition. C. B. Bucknall et al. investigated polyethersulfone as a thermoplastic resin for modifying an epoxy resin composition (British Polymer Journal, (1983), Vol.15 p.71). From this study, they obtained results and conclusions that the cured resin had a structure where micro phases were separated and included a domain where polyethersulfone existed in high concentration. However, the effect of the addition of the polyethersulfone on improving toughness of the cured resin was small regardless of the degree of the phase separation and the composition.
(3) At the 7th International Conference SAMPE European Chapter p.163 (1986), Muraki et al. reported that toughness of the cured resin and impact resistance of CFRP could be improved by modifying an epoxy resin with a thermoplastic resin such as a polyethersulfone. However, the values of toughness described there were still insufficient. They reported that the morphology varied with the composition, but the correlation between the morphology and the toughness could not be clarified.
(4) U.S. Pat. No. 4,656,208 and JP-A-228016/1986 disclosed the incorporation into an epoxy resin composition of a polysulfone oligomer having terminal epoxy reactive functional groups. The cured resin had a microphase separating structure (islands-in-a-sea structure) and the polysulfone existed in a continuous phase with a high concentration. As a result, good heat resistance and high toughness were obtained. J. F. McGrath presented a similar investigation at the 31st SAMPE Symposium p.580 (1986). Toughness of the resin increased with increase in the molecular weight and the amount of loading of polysulfone, but there was a defect that the viscosity of the system accordingly increased and processability decreased. No reference was made to water resistance, but we would estimate that as the water resistance was largely influenced by the water absorption ratio of the epoxy resin, it did not result in any improvement. EP-A-0311349 (1989), discloses the results of an investigation where an amine-terminated polyarylsulfone is incorporated into an epoxy resin. The morphology of the desired cured resin is not clear, as the picture shown there is not clear, but it appears that the morphology varies in dependence upon the amount of SO.sub.2 in the polymer chain of the polyarylsulfone, namely (a) a generally homogeneous structure provided by too small an amount of SO.sub.2, (b) a structure containing separate respective phases, the polyarylsulfone being a continuous phase and the epoxy resin being an island phase, which structure is provided by too great an amount of SO.sub.2 and (c) the desired structure containing separate respective phases of the polyarylsulfone and epoxy resin, but both phases being continuous. The highest toughness was obtained when both the polyarylsulfone phase and the epoxy resin phase had continuous structures. However, we would estimate that the water absorption ratio of this resin system was also largely influenced by the water absorption ratio of the base epoxy resin so that no large improvement would result.
(5) E. M. Yorkgitis et al. reported in Advanced Polymer Science Vol.92 p.79 (1985) that fracture toughness could be improved by introducing a rubbery structure of a siloxane chain in the main chain skeleton of an epoxy resin. However, the effect was not enough and the elastic modulus remarkably decreased. Furthermore, Nonogaki et al. reported in the Preprint of the Society of Polymer Science, Japan Vol.36, No.3, p.739 (1987) that an ethynyl group-terminated siloxane imide oligomer was incorporated in an epoxy resin and the cured product was tough, but the elastic modulus decreased due to the influence of the siloxane skeleton. As described above, in all the conventional methods in which attempts were made to improved toughness defects remained in that in some cases the effects were insufficient and in other cases modulus, heat resistance and processability decreased. There was no method in which high toughness, high elongation and low water absorption could be accomplished while high heat resistance and high modulus were retained. We have conducted extensive investigations in an attempt to provide a thermosetting resin composition having good processability and shelf-life and capable of providing cured products, especially FRPs, having high toughness, high elongation and low internal stress characteristics together with high strength, high modulus, low water absorption and high heat resistance, as well as an excellent ability to maintain these characteristics.
Compositions in accordance with the invention, with which such advantages may be obtained, and cured products produced therefrom are described below. In particular, prepregs in which the compositions are used as matrix resins and cured fiber-reinforced plastics materials produced from such prepregs are described.